Cryogenic device with compact exchanger

ABSTRACT

This cold generation device implements the “Joule-Thomson” expansion principle. It includes a heat exchanger having a fluid under high pressure and under low pressure circulating in counterflow therethrough. The heat exchanger is formed of the stack of pellets ( 5 ) made of a porous material, and particularly a sintered material, forming a cylindrical mandrel, having a capillary ( 10 ) wound at the periphery thereof and in contact therewith, the capillary having the high-pressure fluid circulating therethrough, the low-pressure fluid circulating in counterflow inside of the porous mandrel thus formed.

TECHNOLOGICAL FIELD

The disclosure pertains to the general field of refrigerating machines, and more particularly to cold generation devices intended to allow the operation of certain types of detectors, and more particularly of infrared detectors of cooled type, also called quantum infrared detectors. It more particularly targets devices of the type in question implementing as a cold source the principle of the so-called “Joule-Thomson” expansion.

BACKGROUND

In the specific context of infrared detectors, it is desired, for obvious bulk reasons, to limit the volume of the cryogenic source. Actually, miniature cryogenic machines frequently use the “Joule-Thomson” expansion principle, thus enabling to have a high cryogenic power, and accordingly a fast cooling, particularly of infrared detectors or of electronic components requiring, for their operation, operating at relatively low temperatures.

The performance of such cryogenic machines is known to depend on the efficiency of the heat exchange which occurs between the high-pressure fluid and the low-pressure fluid before the expansion of the fluid occurs. The efficiency of the heat exchange is thus essential.

For this purpose, prior art devices use a Hampson-type counterflow exchanger, where the high-pressure fluid flows in a capillary surrounding a cylindrical sleeve or mandrel, closed by insulating foam. The heat exchange takes place at the periphery of the sleeve, at the level of which the low-pressure fluid circulates in counterflow.

To optimize such a heat exchange, it has been provided to increase the surface area of exchange between the high-pressure fluid and the low-pressure fluid, by providing the capillary with radial fins. If, indeed, the heat exchange surface area is thereby increased, however, the presence of fins, due to their thickness, increases the spacing between two consecutive spirals, and thereby decreases the number of spirals of the capillary for a given length of the mandrel, at least partially neutralizing the desired optimization of the exchange.

For the same purpose, it has already been provided to increase the length of the exchanger, and more particularly the length of the capillary. The issue of the bulk of said exchanger, and thus of the refrigerating machine, then arises.

It has already been provided to decrease the axial conduction in the exchanger, which is inherent to the use of the mandrel and a source of loss of efficiency.

SUMMARY OF THE DISCLOSURE

The disclosed embodiments aim at a device of the type in question enabling both to increase the efficiency of such a device, particularly by decreasing the cooling time of the installation, without altering the bulk of existing devices or, on the contrary, with a constant cooling time, to decrease the bulk of such devices.

For this purpose, the disclosed embodiments provide a cold generation device implementing the “Joule-Thomson” expansion principle, comprising an exchanger having a fluid under high pressure and under low pressure circulating in counterflow therein.

According to the disclosed embodiments, the heat exchanger is formed of the stack of pellets made of a porous material, and particularly a sintered material, forming a cylindrical mandrel having a capillary wound in contact therewith, the capillary having the high-pressure fluid circulating therethrough, the low-pressure fluid circulating in counterflow inside of the porous mandrel thus formed.

Further, a thermally insulating porous fabric, typically made of fiber glass, is interposed between each of the pellets made of sintered material.

In other words, the disclosed embodiments basically comprise replacing the mandrel and the fins of prior art with a stack of porous sintered material, favoring the heat exchange of the low-pressure fluid with the high-pressure fluid circulating in the peripheral capillary in contact with said material.

Such an optimization of the exchange results from the nature of the material forming the mandrel, and further enables to do away with the fins optimizing the heat exchange of prior art, and accordingly enables to optimize the spiral concentration of the capillary having the high-pressure fluid circulating therethrough, and accordingly enables to optimize the compactness of the cold generation device.

Further, due to the interposition, between the pellets of sintered material, of thermally-insulating grids, typically made of fiber glass, which thus do not conduct heat, the axial conduction is decreased and the operation of the cold generation device is accordingly optimized.

Advantageously, the pellets are made up of sintered silver or of sintered copper.

The capillary is made of a metal, typically of copper, of stainless steel, or even of a cupronickel alloy.

According to an advantageous feature, the spirals of the capillary are not in contact with one another. To achieve this, a thermally-insulating yarn, typically made of fiber glass and used as a spacer, is wound together with said capillary. Such a yarn ensures different functions:

thermally insulating two consecutive spirals of the capillary;

thermally insulating said spirals of the external tube or well into which the device is likely to be introduced;

ensuring a tightness of the device with such an external tube or well, forcing the low-pressure fluid to pass through the sintered material pellets, inducing an optimization of the efficiency.

BRIEF DESCRIPTION OF THE DRAWINGS

The way in which the embodiments may be implemented and the resulting advantages will better appear from the following non-limiting description, in relation with the accompanying drawings, among which:

FIG. 1 is a diagram illustrating the principle of the “Joule-Thomson” expansion implemented at the level of the cold generation device.

FIG. 2 is a simplified representation of the device.

FIG. 3 is a view similar to FIG. 2 illustrating the respective circuit of the high-pressure and low-pressure fluid;

FIG. 4 is a simplified representation of a cryostat;

FIG. 5 is a simplified representation in partial sagittal cross-section view of one of the portions of the cryostat of FIG. 4.

DETAILED DESCRIPTION

The operating diagram of a device implementing the “Joule-Thomson” expansion has thus been shown in relation with FIG. 1. The diagram shows the source of high-pressure fluid HP, which fluid may be a gas, typically argon, nitrogen or air, and the return of said fluid after the expansion.

The counterflow heat exchanger between the high-pressure fluid originating from high-pressure source HP and the low-pressure fluid, after expansion at the level of the evaporator (2), an expansion valve (3) being mounted before the evaporator, has been shown by double coil (1). The assembly is integrated within a vacuum enclosure (4).

The core of the exchanger has been shown in FIG. 2. It is formed by the stack of pellets (5), made of porous material, and particularly of a sintered material made up of silver. Silver is indeed a very fine heat conductor and is further easy to sinter. It may also be envisaged to use copper to replace silver.

Typically, the porosity of such pellets is close to 100 nanometers. In other words, the orifices generated by the sintering of the pellets have a typical diameter of 100 nanometers.

Such pellets (5), of generally cylindrical shape, are for example assembled to one another by means of securing rods (6), starting from the high-pressure connector (7), and provided with nuts (8) at their lower base. As a variation, the pellets may be glued together.

According to the contemplated embodiments, the pellets (5) are separated from one another by an intercalary or grid (9) made of a non-conductive porous material, typically formed of a fiber glass woven material. Such intercalaries have a typically 0.3-millimeter thickness. The use of such intercalaries tends to oppose any axial heat conduction, optimizing the surface area of heat exchange between the two flows, respectively at low pressure and high pressure.

The assembly thus formed by the pellets and the intercalaries forms a cylindrical mandrel, having a capillary (10) wound in contact therewith, the high-pressure fluid flowing through the capillary. The capillary is for example made of copper, of stainless steel or of a cupronickel alloy. It typically has an outer diameter of 0.5 millimeter and an inner diameter of 0.3 millimeter.

Due to the porous character of the pellets (5), the low-pressure fluid crosses them and cools them. In turn, due to their good heat conductivity, the pellets cool down the high-pressure fluid which flows through the capillary. Actually, a good thermal contact is necessary between the capillary and the pellets.

The manufacturing of such a device may be carried out as follows.

First, the pellets (5) are formed by means of a mould shaped according to the desired shape of said pellets. The silver powder is poured into the mould, and the temperature of the mould is raised to a temperature lower than the melting temperature of silver, to obtain a simple sintering without causing a melting of the powder.

After having been manufactured, the pellets are stacked by interposing the thermally-insulating elements (9), the latter having an external diameter smaller than or equal to that of the pellets (5), so that they cannot come into contact with the capillary (10).

The pellets and the intercalaries are slipped on the securing rods (6), for example, threaded, and locked by means of the nuts (8). A mandrel is thus formed de facto.

The capillary, for example, made of a cupronickel alloy, is submitted to a treatment comprised of a silver deposition, for example, by electrolysis, if the pellets are made of sintered silver. Such a deposition aims at favoring the subsequent contact with the pellets (5), particularly when said capillary is secured by welding or by soldering. Thus, after the winding of the capillary (10) around the mandrel, the assembly is placed in a furnace to generate the soldering phenomenon.

As a variation, it may be envisaged to consolidate the assembly thus formed by means of a thermally-conductive binder, for example formed of a “solgel”-type glue film filled with metal powder, applied in the capillary/pellet area.

According to an advantageous feature, it is desired to avoid any contact between the consecutive spirals of the capillary, to avoid any thermal bridge between them.

For this purpose, it should be reminded that the device aims at being integrated in a cylindrical well of a cryostat, such as illustrated in FIG. 4. Such a cryostat (11) is conventionally maintained in vacuum. It receives in the enclosure that it defines an infrared detector (12), positioned vertically in line with a window (13) transparent to the radiation to be detected. Finally, it comprises two wells (14) having a device inserted in each of them to generate the cold necessary to the operation of said detector.

FIG. 5 shows a simplified view in partial sagittal cross-section view of one of the wells (14) provided with the device.

Thus, to force the low-pressure fluid, and particularly the low-pressure gas, to cross the porous pellets (5), a yarn (15) made of an insulating material, for example, made of fiber glass or of polyester fibers such as commercialize under trademark Terylene®, bearing between two consecutive spirals of the capillary (10), that is, in the interval separating said spirals, and against the inner wall (16) of the cylindrical well (14), is positioned. The yarn (15) is thus wound along the mandrel, and then secured at its two ends, typically by gluing.

Thus, with the arranging of the yarn (15), it is done away with any thermal bridge between spirals on the one hand, and between the spirals and the well (14).

The consecutive spirals of the capillary (10) are thus thermally insulated from one another. Further, the spirals of the capillary (10) are thermally insulated from the bridge (14).

Finally, the presence of the yarn (15) provides a tightness of the device with respect to said well, forcing the low-pressure fluid to cross the pellets (5) and thus contributing to optimizing the efficiency of the device.

In the specific case of the implementation of the device for an infrared detector, the operating temperature thereof is typically in the range from 77 K to 250 K.

The pressure of the high-pressure fluid is typically in the range from a few tens to a few hundreds of bars.

The device enables to considerably increase the heat exchange surface area as compared with prior art devices, of the type comprising a capillary with fins, typically 1,000 times at constant bulk. It can then easily be understood that the efficiency of such a refrigerating machine is itself increased, or that the bulk of such a refrigerating machine may be significantly decreased, while keeping the same performance as prior art devices. Such results are particularly advantageous in the context of cooled infrared detectors. 

1. A cold generation device implementing the “Joule-Thomson” expansion principle, comprising a heat exchanger having a fluid under high pressure and under low pressure circulating in counterflow therein, wherein the heat exchanger is formed of the stack of pellets made of a porous material, and particularly a sintered material, forming a cylindrical mandrel, having a capillary wound at the periphery thereof and in contact therewith, the capillary having the high-pressure fluid circulating therethrough, the low-pressure fluid circulating in counterflow inside of the porous mandrel thus formed; and wherein a thermally-insulating porous element is interposed between each of the pellets.
 2. The cold generation device of claim 1, wherein the porous thermally-insulating element is formed of a fabric, particularly made of fiber glass.
 3. The cold generation device of claim 1, wherein the pellets have a cylindrical shape, wherein the thermally-insulating intercalary elements have a circular shape, and wherein the diameter of the intercalary elements is smaller than or equal to the external diameter of the pellets.
 4. The cold generation device of claim 1, wherein the pellets are made up of sintered silver or copper.
 5. The cold generation device of claim 1, wherein the capillary is made of metal, particularly of copper, of stainless steel, or of cupronickel alloy.
 6. The cold generation device of claim 5, wherein the capillary receives a silver deposit, which is a function of the nature of the material forming the pellets, said deposit being realized before the winding of the capillary on the mandrel formed by the stack of pellets.
 7. The cold generation device of claim 1, wherein the spirals defined by the winding of the capillary around the mandrel formed by the stack of pellets are not in contact with one another.
 8. The cold generation device of claim 7, wherein a thermally-insulating yarn is wound on the intervals separating the spirals, said yarn being particularly made up of fiber glass. 